Improved Learning And Memory Of Contextual Fear Conditioning And Hippocampal CA1 Long-term Potentiation In Histidine Decarboxylase Knock-out Mice

Histamine is one of the most widely distributed inflammatory substances in the body. Compared with other aminergic systems, the histaminergic system in the CNS gained general acceptance only in 1984, after the immunohistochemical demonstration that the tuberomamillary (TM) nucleus was the sole seat of histaminergic neurons and the origin of the widely distributed histaminergic projections. It is now thought that their efferent fibers project to almost the entire brain, including the hypothalamus, septum, thalamus, cortex, amygdala, and hippocampus. Histamine is synthesised in brain from L-histidine by the enzyme histidine decarboxylase (HDC), and up to now HDC is the only enzyme catalyzed the process. Four histamine receptors have been cloned (H1-H4), and the H1, H2 and H3 histamine receptors are all expressed in distinctive patterns in the brain. The H1 receptor is a 486-491 amino acid protein encoded by an intronless gene, and is coupled to the Gq/11 protein and phospholipase C. The H2 receptor is coupled to Gs and protein kinase A and the protein consists of 358-359 amino acids. The H3 autoreceptor is coupled to Gi/Go, displays a significant constitutive activity, and controls histamine release and synthesis. The H3 receptor regulates the release of several transmitters in brain areas, such as the substantia nigra, amygdala and cerebral cortex.Histamine controls a variety of neurobiological functions and behavioral responses including sleep-wake cycle, water consumption, food, motor activity, and nociception. Histamine is also involved in learning and memory, and many researchers have contributed a lot in this field. However, these studies yielded to contradictory results. For instance, histamine improved and impaired active avoidance conditioning. The HDC-blocker α-iluoromethylhistidine (α-FMH) also has those effects in a radial-maze task. Furthermore, H₁ receptor antagonism has different influence on spatial memory performance and some emotional tasks. There were also discrepancies found with H₂ receptor antagonists and agonists and H3 receptor. The mechanisms underlying these differences seem to be very complex, which may be in part due to the methods used and the approaches selected in the experiments. Most histaminergic agents, and lesions of the TM and hippocampus, not only influence the histaminergic system but also affect non-histaminergic systems in the brain. So, recently histidine decarboxylase knockout (HDC-KO) mice have been developed to study the role of the histaminergic system in learning and other behaviors more specifically and to explain these discrepancies. Previous research showed that HDC-KO mice had improved water maze performance during both hidden and cued platform tasks, but exhibit deficient object discrimination based on temporal relationships. It is proposed that disruption of brain histamine synthesis may actually have bidirectional effects on learning and memory related to the reinforcement contingencies inherent in the task. However, this hypothesis needs further confirmation. Furthermore, these studies have not explained the mechanisms of the enhancement of learning and memory in the behavioral tasks. Contextual and spatial learning and memory is closely related to the synaptic plasiticity, and it is still unclear whether long-term histamine deficiency influences the synaptic plasiticity during the behavioral performance in HDC-KO mice.In the present study, we investigated the change of learning and memory in hippocampal-dependent contextual fear conditioning, and the synaptic plasiticity in hippocampal CA1 region using electrophysiological recording to explain the mechanisms of behavioral performance in HDC-KO mice.Part I Contextual fear conditioning in HDC-KO micePavlovian fear conditioning has emerged as a leading behavioral paradigm for studying the neurobiological basis of learning and memory. It can be divided into contextual fear conditioning and cued fear conditioning according to the conditioned sitmulus. Contextual but not cued fear memory is dependent on the hippocampus. In this paradigm, mice were given the opportunity to associate both a tone (cue) and the apparatus (context) with footshock in a single training trial. Memory was assessed by scoring the percent time mice spent immobile (freezing, a fear reaction) upon reexposure either to the context or the cue in a distinct context. In this part of investigation, we used contextual fear conditioning to examine learning and memory ability in HDC-KO mice. The data showed that mice lacking histamine exhibited improved contextual fear memory, and this improvement was maintained for a long period from 2h to 14d. The intracerebroventricular injection of histamine before and immediately after training reversed the improvement, while the injection before testing had on effects. HDC-KO mice also showed enhancement of cued fear compared with the wildtype (WT) mice. The results indicated that contextual fear memory increased in HDC-KO mice from 2h to 14d after training, and long-term histamine deficiency may upregulate hippocampal acquisition and consolidation of contextual fear. While these influences may be not specific to the hippocampus, amygdala may also be involved in the process. Part II synaptic plasiticity in the hippocampal CA1 region inHDC-KO miceHippocampus plays an important role in synaptic plasiticity. Physiologial activity-dependent long-term changes in synaptic transmission, as long-term potentiation (LTP) are thought to be the substrate of learning and memory. Contextual and spatial learning and memory are closely related to the LTP in the synapse from the Schaffer collateral pathway to CA1 cells. We found that long-term histamine deficiency increased the acquisition and consolidation of contextual fear in HDC-KO mice. In this part, we investigated the synaptic plasiticity in hippocampal CA1 region to explain the behavioral performance in HDC-KO mice. The data showed that there was no difference in input-out curve and LTD between the two genotypes. While under normal conditions, LTP was stronger in HDC-KO mice than that in WT mice 120 min after induction, and 10 μM histamine perfusion reversed this enhancement. Paired-pulse facilitation (PPF) was significantly decreased after the LTP induction in HDC-KO mice compared with that in WT mice. 1 day after training, LTP in HDC-KO mice was decreased compared with WT mice, which implied involvement of activity-dependent LTP in associative learning and memory. And HPLC analysis showed that hippocampal glutamate content increased in HDC-KO mice 1d and 4d after training. The results indicated that LTP increase in HDC-KO mice, and presynaptic gluatamate release may be also involved during the process. The changes in synaptic plasiticity in the hippocampus may contribute to the improvement in learning and memory. Summary1. Contextual fear memory increased in HDC-KO mice from 2h to 14d after training, and long-term histamine deficiency may upregulate hippocampal acquisition and consolidation of contextual fear. While these influences may be not specific to the hippocampus, amygdala may also be involved in the process.2. The electrophysiological recording showed that there was no difference in input-out curve between the two genotypes, which indicated the basal synaptic transmission do not change in HDC-KO mice. And long-term histamine deficiency may increase LTP in HDC-KO mice, while LTD did not change. Hippocampal glutamate content increase 1d and 4d after training, and PPF decrease after LTP induction indicated that presynaptic glutamate release may be involved in the enhancement of contexutal fear and LTP.3. LTP in HDC-KO mice was significantly decreased compared with that in WT mice 1d after training. These results indicated that the synaptic efficiency increases 1d after contextual fear training, and this phenomenon may be closely related to activity-dependent LTP in associative learning and memory.4. In the present study, we provide additional evidence that histamine is involved in acquisition and consolidation of contextual fear memory in a hippocampal -dependent manner. Fourthermore, we demonstrated that long-term histamine deficiency may contribute to improved contextual fear memory and spatial memory in HDC-KO mice. Compared with the effects of exogenous histamine on learning and memory, we elucidated the relationship between endogenous histamine deficiency and synaptic plasiticity, which provide a new aspect in this field.